In general, when a digital color copy machine performs copying of a color image, the original is first scanned in order to obtain image signals corresponding to the three colors yellow, magenta, and cyan. Then, based on these image signals, a copy image is formed using toners of the three foregoing colors. Further, in digital color copy machines using three colored toners, the colored toners are also used for black pixels.
However, when recreating black pixels using three colored toners, toner consumption is greater than when using black toner, and there are problems such as blurring of the outline of the image due to slight misalignment of the three colors. These problems are especially marked when copying a monochrome original such as a page of characters.
For this reason, some digital color copy machines now on the market are provided with a color type determining device whose function is to pre-scan the original to read the image data thereof, and, based on the image data, to automatically distinguish generally whether the original is a color image including colored information, or a monochrome original made up solely of non-colored information.
In this kind of color type determining device, generally, the image data read is first stored in an image memory, and the stored data is broken down by pixel into yellow (hereinafter referred to simply as "Y"), magenta (hereinafter referred to simply as "M"), and cyan (hereinafter referred to simply as "C"). Then it is determined whether the original is a color original or monochrome original based on a ratio among the Y, M, and C components. Generally, this kind of color type determining device, based on the number (area ratio) of color pixels in the image data, decides that an original is a color original when the number of color pixels is greater than a predetermined number.
Then, if it is decided that the original is a monochrome original made up solely of non-colored information, copying is performed using black toner alone, and if it is decided that it is a color original including colored information, color copying is performed using toners of the three colors Y, M, and C, or using toners of the four colors Y, M, C, and black.
However, with the foregoing conventional structure, in order to determine whether the original is color or monochrome, it is necessary to first store the read image data in an image memory, and thus an expensive, high capacity image memory is necessary. This gives rise to the problem of increase of the cost of manufacturing the copying machine. Further, with the foregoing conventional structure, since the color type of the original is determined after first storing the read image data in the image memory, it is not possible to automatically determine from the read image data in real time whether the original is color or monochrome, and this has the drawback that high-speed processing is not possible.
In this connection, Japanese Unexamined Patent Publication No. 4-282968/1992 (Tokukaihei 4-282968) discloses a method of determining whether an original is color or monochrome based on a number of color blocks outputted for each line. In this conventional art, it is first determined whether each pixel is color or monochrome, then whether each line is color or monochrome, and then whether the entire original is color or monochrome.
However, with the art disclosed in the foregoing publication, whether a line is color or monochrome is determined based on whether the pixels thereof are color pixels or monochrome pixels, which gives rise to the problem that it is difficult to reflect general characteristics of the line, such as edge portions, relationships among pixels, etc.
In other words, in the color type determination method disclosed in Japanese Unexamined Patent Publication No. 4-282968/1992, when more than a predetermined number of successive color pixels are distinguished in a line, that successive area is treated as a color block, and whether or not the line is a color line is determined based on the number of color blocks in the line.
However, with the foregoing color type determination method, since the basic unit is a predetermined number of successive color pixels, a condition of color type determination is that the image includes a color area of a certain extent. Consequently, images having color areas of comparatively small extent, such as color images made up of dots, are outside the scope of color type determination.
With the art disclosed in the foregoing publication, in order to determine the color type of images having color areas of comparatively small extent, it is sufficient to reduce the predetermined number of successive color pixels, which is the standard for determining the color blocks. However, in this case, color noise, etc. may be treated as a color block, and the precision of color type determination is reduced. Incidentally, color noise is noise which occurs primarily at edge portions of monochrome lines due to the precision of the CCD, vibration or misalignment of devices, etc.